Formation of Supermassive Black Holes in Galactic Bulges: A Rotating Collapse Model Consistent with theMBH‐σ Relation

Abstract

Motivated by the observed correlation between black hole masses MBH and the velocity dispersion σ of host galaxies, we develop a theoretical model of black hole formation in galactic bulges (this paper generalizes an earlier ApJ Letter). The model assumes an initial state specified by a uniform rotation rate Ω and a density distribution of the form ρ = a/2πGr2 (so that aeff is an effective transport speed). The black hole mass is determined when the centrifugal radius of the collapse flow exceeds the capture radius of the central black hole (for Schwarzschild geometry). This model reproduces the observed correlation between the estimated black hole masses and the velocity dispersions of galactic bulges, i.e., MBH ≈ 108 M☉(σ/200 km s-1)4, where σ = aeff. To obtain this normalization, the rotation rate Ω ≈ 2 × 10-15 rad s-1. The model also defines a bulge mass scale MB. If we identify the scale MB with the bulge mass, the model determines the ratio μB of black hole mass to the host mass: μB ≈ 0.0024(σ/200 km s-1), again in reasonable agreement with observed values. In this scenario, supermassive black holes form quickly (in ~105 yr) and are born rapidly rotating (with a/M ~ 0.9). This paper also shows how these results depend on the assumed initial conditions; the most important quantity is the initial distribution of specific angular momentum in the precollapse state.